Sister's fiance is rude to us and it's driving a wedge between the family

My sister Lucy has been with Tom for 8 years. They both live a couple of hours away from the majority of mine and Lucy's family. They are also in their late 20s if it makes any difference.

In a few months my sister and Tom are getting married and it's a huge event. Lucy has been talking about it for the last 2 years, making preparations and planning it all. She is excited and I would be too, if it wasn't for Tom.

So Tom has always been erm... quiet. He's obviously met mine and Lucy's parents many times however every single time he spends time with them he is extremely quiet, will not say hello or greet anyone. He simply sulks about, looks at his phone or even goes into the spare room to sleep. One Christmas he spent a lot of time in the other room away from everyone else. At dinner he hardly says a word, and most upsetting for me, he will not even thank our parents for the dinner NOR even say goodbye when he leaves. Everyone kind of brushes it off as it being part of his personality. And mostly we accept it.

To be honest however I have never really had a good feeling about him. Lucy always makes excuses for him and says he is anxious. But frankly I don't believe it. The first time he met my own partner Jack, we all went to a pub. Instead of everyone getting to know each other over a few drinks, some conversations and laughter, he sat awkwardly staring at the table, and left early without saying a word. No goodbye, nothing. Lucy made her excuses for him and the get together was over.

I would understand if I thought he truly had an anxiety issue, however I find it hard to believe that in the 8 years of being with my sister he has never been able to relax or open up around any of us. To add to this, myself and my mum once caught him laughing and talking to his colleagues at work. In this moment he appeared totally confident and almost like a different person. With his own family he is also seemingly a different person. He not once has acted that comfortable or confident around mine ane Lucy's family. Our parents are kind to him, treat him like part of the family. We all tried to make a connection with him. I see no reason why he wouldn't like us.

To make things worse, one experience really changed how I see him. Lucy and Tom once came to spend new years with me and my partner Jack. During the taxi ride to the pub, Tom weirdly and out of character persuaded Jack to get into the front of the taxi. He then sat in the back between me and my sister and proceeded to subtly rub my leg with his finger during the ride. I brushed it off as a mistake, despite feeling a bit odd about it. On the way home, after a few drinks, the same happened again. Tom purposely positioned himself between myself and Lucy, and again I felt his hand stroking my upper leg. I told everyone about this except Lucy. I didn't feel right bringing it up, due to her extreme defensiveness about him. I never really forgot it happened, but I chose to move on.

Fast forward a few weeks ago... Myself and Jack decided to visit my sister for the first time in a long while, and upon meeting Tom and Lucy, Tom decided to not say hello again. Lucy smiled and was happy to see us but Tom sat, staring at the table, not saying a word. Not asking us how the drive was, not even a simple Hello.

I tried to ask what's wrong but Lucy quickly jumped in and defended Tom. "He doesn't HAVE to speak to you". I found this odd seeing as we had came to visit them, and were going to be staying in their house. It created an awkward atmosphere.

A few days later Tom and Jack are downstairs and Jack tries to strike up a conversation with him. One word answers. Obviously not wanting to engage with Jack. It made Jack feel disliked by him.

I tried to subtly speak to my sister about his behaviour but she immediately shut me down, refusing to discuss the matter. In fact she shouted loudly (within earshot of Tom) that I am not allowed to speak badly of him in his house, and that she won't hear another word about it. I not once said a bad word about him, only questioning if he is ok, and why he didn't want to speak with Jack.

Overall, I am concerned. I'm even reconsidering attending the wedding. Nothing I say can get through to my sister and I just don't understand why Tom doesn't seem to get on with us. His weird behaviour in the taxi also leads me to feel unsure about him. I guess we have to accept him as he is, but I feel like I'm drifting away from my sister due to him. I don't feel like I want to visit their house anymore either.

Sorry for a long rant. It's probably not worded well either.